Ink jet printing processes are mainly of two types: continuous stream and drop-on-demand.
In continuous stream ink jet printing, a continuous ink stream is emitted under pressure through a nozzle. The stream breaks up into droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed the individual droplets are deposited on the recording sheet, otherwise they are directed to a collecting vessel. This is done for example by charging unnecessary droplets in accordance with digital data signals and passing them through a static electric field which adjusts the trajectory of these droplets in order to direct them to the collecting vessel. The inverse procedure may also be used wherein uncharged droplets are collected in the vessel.
In the non-continuous process, or the so-called “drop-on-demand” process, a droplet is generated and expelled from the nozzle in accordance with digital data signals only if a specific location on the recording sheet has to be printed.
The printing speed of modern ink jet printers is ever increasing for economical reasons. Recording sheets suitable for these printers therefore need to absorb the inks very quickly. Especially suitable are recording sheets containing nanoporous inorganic compounds, preferably oxides such as aluminum oxides or silicon dioxide, or oxide/hydroxides such as aluminum oxide/hydroxides. Such recording sheets are known as “nanoporous” recording sheets.
Nanoporous recording sheets for ink jet printing containing nanocrystalline, nanoporous aluminum oxide or aluminum oxide/hydroxide are described for example in patent application EP 0,298,424.
Image quality is improved in the case where the nanocrystalline, nanoporous aluminum oxide or aluminum oxide/hydroxide contains one or more elements of the rare earth metal series of the periodic system of the elements, as described for example in patent application EP 0,875,394.
It is an advantage if the positive charge of the surface of nanocrystalline, nanoporous aluminum oxide or aluminum oxide/hydroxide is increased by a treatment with aluminum chlorohydrate, as described for example in patent application EP 1,437,228.
Recording sheets containing nanocrystalline, nanoporous aluminum oxide or aluminum oxide/hydroxide show excellent image quality, very high gloss and excellent transparency due to the small particle size of the aluminum oxide or aluminum oxide/hydroxide. Ink absorption capacity is only sufficiently high with a sufficiently high quantity of aluminum oxide or aluminum oxide/hydroxide because its pore volume is relatively low. Because high quantities of aluminum oxide or aluminum oxide/hydroxide are needed in such recording sheets and the price of aluminum oxide or aluminum oxide/hydroxide is high, manufacturing costs of such recording sheets are high. Manufacturing speed is relatively low because high quantities of coating solutions need to be coated and dried for the ink-receiving layers.
Patent application DE 10,020,346 describes a recording sheet which contains silicon dioxide prepared in the gas phase with a size of the primary particles of at most 20 nm, wherein the surface of the silicon dioxide has been modified by a treatment with aluminum chlorohydrate.
Patent application WO 00/20,221 describes the reaction of silicon dioxide prepared in the gas phase with aluminum chlorohydrate. The modified silicon dioxide is incorporated afterwards into an ink-receiving layer of nanoporous recording sheets for ink jet printing.
Patent application WO 02/094,573 describes the use of silicon dioxide prepared in the gas phase in recording sheets for ink jet printing, wherein the surface of the silicon dioxide has been modified by a treatment with aminoorganosilanes.
Patent application WO 01/05,599 describes the use of silicon dioxide pigments in recording sheets for ink jet printing, wherein the surface of the silicon dioxide has been modified by a treatment with cationic aminoorganosiloxanes.
Patent application EP 0,983,867 describes the use of colloidal silicon dioxide in recording sheets for ink jet printing, wherein the surface of the silicon dioxide has been modified by a treatment with silanes of general formula (R1)nSi(OR2)4-n, wherein at least one of the substituents R1 contains an amino group.
Patent application EP 1,655,348 describes a method of surface modification of nanoporous silicon dioxide, wherein the silicon dioxide is modified by a treatment with the reaction products of at least one aminoorganosilane with a compound of trivalent aluminum, for example aluminum chlorohydrate. The modified nanoporous silicon dioxide is incorporated afterwards into an ink-receiving layer of a nanoporous recording sheet for ink jet printing.
Nanoporous recording sheets for ink jet printing, having coated onto an ink-receiving layer containing nanoporous silicon dioxide with a pore radius between 4 nm and 25 nm, an ink-receiving layer containing nanocrystalline, nanoporous aluminum oxide/hydroxide, for example pseudo-boehmite, are described for example in patent application EP 0,631,013.
Recording sheets containing nanoporous silicon dioxide have an excellent ink absorption capacity even with a low quantity of nanoporous silicon dioxide because of its high pore volume. Because relatively low quantities of silicon dioxide are needed in such recording sheets and the price of silicon dioxide is relatively low, manufacturing costs of such recording sheets are quite low. Manufacturing speed is high because relatively low quantities of coating solutions need to be coated and dried for the ink-receiving layers. However, image quality as well as transparency of such recording sheets, are not very good.
Nanocrystalline, nanoporous aluminum oxide or aluminum oxide/hydroxide has a pore volume, which is lower by a factor of 1.4 to 2.0 than the pore volume of nanoporous silicon dioxide. Therefore, the quantity of nanocrystalline, nanoporous aluminum oxide or aluminum oxide/hydroxide needed for the absorption of a fixed amount of aqueous inks is higher by a factor of 1.4 to 2.0 than in the case of nanoporous silicon dioxide.
There is therefore a need to improve, in recording sheets for ink jet printing containing nanoporous inorganic compounds, in addition to ink absorption capacity, speed of ink absorption, water fastness, light stability and the like, in particular the image quality and the gloss with dye based inks and with pigment based inks. There is also a need to reduce manufacturing costs.